Sustained dysregulation of cytosolic calcium concentration may be a proximal common denominator of irreversible neuronal injury. Several lines of evidence support the notion that accumulation of calcium occurs during acute stroke and trauma, as well as in the setting of chronic neurodegenerative processes, such as Alzheimer's and Parkinson's diseases. Delineation of whether substantial increases of cytosolic calcium consistently precede neuronal death, and whether this may be modified by protective maneuvers, is essential for establishing a pathogenic role for calcium. However, it has traditionally been difficult to follow neuronal calcium changes in the intact animal brain, and little information exists on changes in neuronal calcium concentration during brain injury. The objective of this proposal is to quantify the disturbances in cerebral calcium homeostasis that occur during and after ischemia in the living rat brain, and to correlate these changes to neuronal injury on the single cell level. Using laser-activated confocal imaging of intraneuronal calcium, it proposed to test the postulate that late disturbances in calcium homeostasis that follow ischemia mediate neuronal death. This will be performed by asking: 1. Do distinct loci of cellular calcium entry yield differential effects on neuronal viability during and after ischemia? 2. Is the increase of cytosolic calcium necessary or sufficient to promote irreversible neuronal injury? 3. Will earlier determination of neuronal death in vivo improve therapeutic efforts, by allowing therapy to be more precisely directed? To address the latter question, the investigators will employ a new approach that they have developed for the determination of cell viability in vivo, the confocal imaging of nuclear staining with a fluorescent DNA marker, propidium iodide. This approach will permit the mapping of neuronal calcium concentration and viability simultaneously, on the single cell level in vivo. Through this approach, the investigators intend to define the temporal relation of intraneuronal calcium dysregulation and cellular viability to a far more precise degree than current approaches allow. By doing so, it is hoped to provide a definition of the temporal window following cell injury but before loss of viability, during which effective neuroprotective therapy may be directed following traumatic and ischemic brain injury.